Ion exchange membrane

ABSTRACT

The ion exchange membrane according to the present invention comprises a layer A comprising a fluorine-containing polymer having a sulfonic acid group and a layer B comprising a fluorine-containing polymer having a carboxylic acid group, wherein a ratio of an ion cluster diameter of the layer B after electrolysis under the following electrolysis conditions to the ion cluster diameter of the layer B before the electrolysis [(the ion cluster diameter of the layer B after the electrolysis)/(the ion cluster diameter of the layer B before the electrolysis)] is 0.83 to 0.95: 
     (Electrolysis Conditions) 
     in a zero-gap base electrolytic cell where the ion exchange membrane is disposed between an anode chamber to which a 3.5 N aqueous sodium chloride solution is supplied and a cathode chamber to which a 10.8 N aqueous sodium hydroxide solution is supplied, electrolysis is performed for 7 days under conditions having a temperature of 85° C. and a current density of 6 kA/m 2 .

TECHNICAL FIELD

The present invention relates to an ion exchange membrane.

BACKGROUND ART

Fluorine-containing ion exchange membranes have excellent heatresistance, chemical resistance, and the like, and are used in variousapplications as electrolytic diaphragms for alkali chlorideelectrolysis, ozone producing electrolysis, fuel cells, waterelectrolysis, hydrochloric acid electrolysis, and the like.

Among these, in alkali chloride electrolysis where chlorine and alkalihydroxide are produced in particular, the ion exchange membrane processis primarily used in recent years. The ion exchange membrane used in theelectrolysis of alkali chloride is required to have variouscharacteristics. For example, required are characteristics such aselectrolytic performance that electrolysis can be performed at a highcurrent efficiency and a low electrolytic voltage, and the concentrationof impurities (such as alkali chloride in particular) contained in theproduced alkali hydroxide is low, as well as membrane strength and likecharacteristics that the membrane strength is so high that no damage isincurred during membrane handling and electrolysis. In addition, whilethe electrolytic performance and the membrane strength of an ionexchange membrane are in a trade-off relationship, there are demands forthe development of an ion exchange membrane having both at high levels.

Patent Literature 1 discloses an ion exchange membrane consisting of atleast two layers, i.e., a fluorine-containing polymer layer having asulfonic acid group and a fluorine-containing polymer layer having acarboxylic acid group.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Publication No.2001-323084

SUMMARY OF INVENTION Technical Problem

However, the ion exchange membrane described in Patent Literature 1 hasroom for further improvement on simultaneously improving membranestrength and electrolytic performance.

The present invention has been conceived in view of the problems of theconventional art described above, and an object of the present inventionis to provide an ion exchange membrane having both excellent membranestrength and excellent electrolytic performance.

Solution to Problem

As a result of having conducted diligent research to solve the aboveproblems, the present inventors found that electrolytic performance isdramatically improved by controlling ion clusters present in an ionexchange membrane to shrink during electrolysis such that the ioncluster diameter after electrolysis of the ion exchange membrane isreduced to a predetermined ratio relative to the ion cluster diameterbefore electrolysis, and accomplished the present invention.

That is to say, the present invention is as set forth below.

[1]

An ion exchange membrane comprising:

a layer A comprising a fluorine-containing polymer having a sulfonicacid group; and

a layer B comprising a fluorine-containing polymer having a carboxylicacid group, wherein

a ratio of an ion cluster diameter of the layer B after electrolysisunder the following electrolysis conditions to the ion cluster diameterof the layer B before the electrolysis [(the ion cluster diameter of thelayer B after the electrolysis)/(the ion cluster diameter of the layer Bbefore the electrolysis)] is 0.83 to 0.95:

(Electrolysis conditions)

in a zero-gap base electrolytic cell where the ion exchange membrane isdisposed between an anode chamber to which a 3.5 N aqueous sodiumchloride solution is supplied and a cathode chamber to which a 10.8 Naqueous sodium hydroxide solution is supplied, electrolysis is performedfor 7 days under conditions having a temperature of 85° C. and a currentdensity of 6 kA/m².

[2]

The ion exchange membrane according to [1], wherein

the ion cluster diameter of the layer B before the electrolysis is 2.5to 4.0 nm; and

the ion cluster diameter of the layer B after the electrolysis is 2.0 to3.3 nm.

[3]

The ion exchange membrane according to [1] or [2], wherein a sum of athickness of the layer A and a thickness of the layer B before theelectrolysis is 55 μm or more.

[4]

The ion exchange membrane according to any of [1] to [3], wherein theion cluster diameter of the layer A before the electrolysis is 3.0 to4.5 nm.

[5]

The ion exchange membrane according to any of [1] to [4], wherein

a thickness of the layer A before the electrolysis is 50 to 180 μm; and

a thickness of the layer B before the electrolysis is 5 to 20 μm.

[6]

The ion exchange membrane according to any of [1] to [5], wherein

the layer A comprises a polymer of a compound represented by thefollowing formula (2); and

the layer B comprises a polymer of a compound represented by thefollowing formula (3):

CF₂═CF—(OCF₂CYF)_(a)—O—(CF₂)_(b)—SO₂F  (2)

wherein a represents an integer of 0 to 2, b represents an integer of 1to 4, and Y represents —F or —CF₃; and

CF₂═CF—(OCF₂CYF)_(c)—O—(CF₂)_(d)—COOR  (3)

wherein c represents an integer of 0 to 2, d represents an integer of 1to 4, Y represents —F or —CF₃, and R represents —CH₃, —C₂H₅, or —C₃H₇.[7] An electrolytic cell comprising the ion exchange membrane accordingto any of [1] to [6].

Advantageous Effects of Invention

The ion exchange membrane of the present invention has excellentmembrane strength and electrolytic performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of one example of an ionexchange membrane of the present embodiment.

FIG. 2 is a schematic view of one example of an electrolytic cell of thepresent embodiment.

DESCRIPTION OF EMBODIMENT

Below, an embodiment for carrying out the present invention (hereinafterreferred to as “the present embodiment”) will now be described indetail. The present invention is not limited to the present embodimentbelow, and can be carried out after making various modifications withinthe scope of the present invention.

The ion exchange membrane of the present embodiment has a layer Acontaining a fluorine-containing polymer having a sulfonic acid group(hereinafter sometimes simply referred to as “layer A”), and a layer Bcontaining a fluorine-containing polymer having a carboxylic acid group(hereinafter sometimes simply referred to as “layer B”), wherein theratio of the ion cluster diameter of the layer B after electrolysisunder the following electrolysis conditions (1) to the ion clusterdiameter of the layer B before the electrolysis [(the ion clusterdiameter of the layer B after electrolysis)/(the ion cluster diameter ofthe layer B before electrolysis)] is 0.83 to 0.95. Here, theelectrolysis conditions (1) are defined that electrolysis is performedfor 7 days under conditions having a temperature of 85° C. and a currentdensity of 6 kA/m² in a zero-gap base electrolytic cell where the ionexchange membrane is disposed between an anode chamber to which a 3.5 Naqueous sodium chloride solution is supplied and a cathode chamber towhich a 10.8 N aqueous sodium hydroxide solution is supplied. Being thusconfigured, the ion exchange membrane of the present embodiment hasexcellent film strength and electrolytic performance. Below,electrolysis under the above electrolytic conditions (1) may be simplyreferred to as “electrolysis”. In the present specification, the“zero-gap” means a state where the ion exchange membrane is in contactwith both the cathode and the anode in an electrolytic layer (a statewhere the distance between the ion exchange membrane and the anode andthe distance between the ion exchange membrane and the cathode arezero), and these components may be in a state where the entire surfaceof the electrode (the anode or cathode) is in contact with the ionexchange membrane or may be in a state where a certain point on theelectrode surface is in contact with the ion exchange membrane.

FIG. 1 shows a schematic cross-sectional view of one example of theconfiguration of the ion exchange membrane of the present embodiment. Inthe ion exchange membrane of the present embodiment, the layer A (4)containing a fluorine-containing polymer having a sulfonic acid groupand the layer B (5) containing a fluorine-containing polymer having acarboxylic acid group are laminated, and there are reinforcement corematerials 3 and continuous holes 2 a and 2 b inside the membrane.Normally, the layer A (4) containing a fluorine-containing polymerhaving a sulfonic acid group is disposed on the anode side (α) of theelectrolytic layer, and the layer B (5) containing a fluorine-containingpolymer having a carboxylic acid group is disposed on the cathode side(β) of the electrolytic layer. The membrane surface has coating layers 6and 7. In FIG. 1, the continuous hole 2 a and the reinforcement corematerials 3 are formed perpendicular to the paper, and the continuoushole 2 b is formed parallel to the paper. That is to say, the continuoushole 2 b formed parallel to the paper is formed in a directionsubstantially perpendicular to the reinforcement core materials 3. Thecontinuous holes 2 a and 2 b may have portions 8 that appear on theanode-side surface of the layer A. As shown in FIG. 1, the ion exchangemembrane of the present embodiment is preferably laminated such that thesurface of the layer A and the surface of the layer B are in contact.Hereinafter, the layer A and the layer B may be collectively referred toas a membrane body.

[Layer A]

The layer A contained in the ion exchange membrane of the presentembodiment contains a fluorine-containing polymer A having a sulfonicacid group (hereinafter sometimes simply referred to as a “polymer A”)and, preferably, consists of the polymer A. Here, “thefluorine-containing polymer having a sulfonic acid group” refers to afluorine-containing polymer having a sulfonic acid group or a sulfonicacid group precursor that can become a sulfonic acid group byhydrolysis. Other than the polymer A, the layer A may contain a polymerB, which will be described below, in a range of less than 20% by massbased on 100% by mass of the layer A, and preferably contains thepolymer A in an amount of 80% by mass or more based on 100% by mass ofthe layer A.

The fluorine-containing polymer A having a sulfonic acid group, whichconstitutes the layer A, can be produced by, for example, copolymerizinga monomer of a first group and a monomer of a second group below, orhomopolymerizing a monomer of a second group. In the case of being acopolymer, the polymer A may be a block polymer or may be a randompolymer.

The monomer of the first group is not particularly limited and is, forexample, a vinyl fluoride compound.

The vinyl fluoride compound is preferably a compound represented by thefollowing general formula (1):

CF₂═CX₁X₂  (1)

wherein X₁ and X₂ each independently represent —F, —Cl, —H, or —CF₃.

The vinyl fluoride compound represented by the above general formula (1)is not particularly limited, and examples include vinyl fluoride,tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride,trifluoroethylene, and chlorotrifluoroethylene.

In particular, in the case of using the ion exchange membrane of thepresent embodiment as a membrane for alkali electrolysis, the vinylfluoride compound is preferably a perfluoro monomer, more preferably aperfluoro monomer selected from the group consisting oftetrafluoroethylene and hexafluoropropylene, and even more preferablytetrafluoroethylene (TFE).

The monomers of the first group may be used singly or in combinations oftwo or more.

The monomer of the second group is not particularly limited and is, forexample, a vinyl compound having a functional group that can beconverted into a sulfonic acid-type ion exchange group.

The vinyl compound having a functional group that can be converted intoa sulfonic acid-type ion exchange group is preferably a compoundrepresented by the following general formula (2):

CF₂═CF—(OCF₂CYF)_(a)—O—(CF₂)_(b)—SO₂F  (2)

wherein a represents an integer of 0 to 2, b represents an integer of 1to 4, and Y represents —F or —CF₃.

In formula (2), when a is 2, a plurality of Y are mutually independent.

The monomer of the second group is not particularly limited, andexamples include monomers shown below:

CF₂═CFOCF₂C F₂SO₂F,CF₂═CFOCF₂CF(CF₃) OCF₂CF₂SO₂F,CF₂═CFOCF₂CF(CF₃) OCF₂CF₂CF₂SO₂F,CF₂═CF(CF₂)₂SO₂F,CF₂═CFO [CF₂CF(CF₃)O]₂CF₂CF₂SO₂F, andCF₂═CFOCF₂CF(CF₂OCF₃) OCF₂CF₂SO₂F.

Among these, CF₂═CFOCF₂CF(CF₃) OCF₂CF₂CF₂SO₂F and CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F are preferable.

The monomers of the second group may be used singly or in combinationsof two or more.

The variety of combinations of monomers constituting the polymer A, andtheir ratio, degree of polymerization, and the like are not particularlylimited. The polymer A contained in the layer A may be a single polymeror a combination of two or more. The ion exchange capacity of thefluorine-containing polymer A having a sulfonic acid group can beadjusted by changing the ratio of monomers represented by the abovegeneral formulae (1) and (2).

The layer A may be a single layer, or may be composed of two or morelayers, according to the composition of the constituting polymer A.

When the layer A is a single layer, the thickness thereof is preferably50 μm or more and 180 μm or less, and more preferably 80 μm or more and160 μm or less. When the thickness of the layer A is within the aboverange, the strength of the membrane body tends to be more increased.

In the present specification, when the layer A has a two-layerstructure, the layer on the side that is brought into contact with theanode is a layer A-1, and the layer on the side that is brought intocontact with the layer B is a fluorine-containing polymer layer A-2.Here, it is preferable that the fluorine-containing polymer that formsthe layer A-1 (also referred to as a “fluorine-containing polymer A-1”)and the fluorine-containing polymer that forms the layer A-2 (alsoreferred to as a “fluorine-containing polymer A-2”) have differentcompositions. The thickness of the layer A-1 is preferably 10 μm or moreand 60 μm or less. The thickness of the layer A-2 is preferably 30 μm ormore and 120 μm or less, and more preferably 40 μm or more and 100 μm orless. When the thicknesses of the layer A-1 and the layer A-2 are withinthe above ranges, the strength of the membrane body can be sufficientlymaintained. The total thickness of the layer A-1 and the layer A-2 ispreferably 50 μm or more and 180 μm or less, and more preferably 80 μmor more and 160 μm or less. When the layer A is composed of two or morelayers, the layer A may be formed by laminating two or more films thatare composed of polymers A having different compositions.

[Layer B]

The layer B contained in the ion exchange membrane of the presentembodiment contains a fluorine-containing polymer B having a carboxylicacid group (hereinafter sometimes simply referred to as a “polymer B”).Here, “the fluorine-containing polymer having a carboxylic acid group”refers to a fluorine-containing polymer having a carboxylic acid groupor a carboxylic acid group precursor that can become a carboxylic acidgroup by hydrolysis. The layer B may contain a component other than thepolymer B in a range of less than 10% by mass based on 100% by mass ofthe layer B, preferably contains the polymer B in an amount of 90% bymass or more based on 100% by mass of the layer B, and particularlypreferably contains the polymer B in an amount of 100% by mass. Examplesof the component that may be contained in the layer B other than thepolymer B include, but are not limited to, metal chlorides such aspotassium chloride.

The fluorine-containing polymer having a carboxylic acid group, whichconstitutes the layer B, can be produced by, for example, copolymerizinga monomer of the above first group and a monomer of a third group below,or homopolymerizing a monomer of the third group. In the case of being acopolymer, the polymer B may be a block copolymer or may be a randompolymer.

The monomer of the third group is not particularly limited and is, forexample, a vinyl compound having a functional group that can beconverted into a carboxylic acid-type ion exchange group.

The vinyl compound having a functional group that can be converted intoa carboxylic acid-type ion exchange group is preferably a compoundrepresented by the following general formula (3):

CF₂═CF—(OCF₂CYF)_(c)—O—(CF₂)_(d)—COOR  (3)

wherein c represents an integer of 0 to 2, d represents an integer of 1to 4, Y represents —F or —CF₃, and R represents —CH₃, —C₂H₅, or —C₃H₇.

In general formula (3), when c is 2, a plurality of Y are mutuallyindependent. In the above general formula (3), it is preferable that Yis —CF₃, and R is —CH₃.

In particular, when the ion exchange membrane of the present embodimentis used as an ion exchange membrane for alkali electrolysis, it ispreferable to use a perfluoro monomer as a monomer of at least the thirdgroup. Note that the alkyl group (see R above) in the ester group islost from the polymer upon hydrolysis, and thus the alkyl group (R) doesnot need to be a perfluoroalkyl group. Among these, while the monomer ofthe third group is not particularly limited, for example, monomers shownbelow are more preferable:

CF₂═CFOCF₂CF(CF₃)OCF₂COOCH₃,CF₂═CFOCF₂CF(CF₃)O(CF₂)₂COOCH₃,CF₂═CF [OCF₂CF(CF₃)]₂O(CF₂)₂COOCH₃,CF₂═CFOCF₂CF(CF₃)O(CF₂)₃COOCH₃,CF₂═CFO(CF₂)₂COOCH₃, andCF₂═CFO(CF₂)₃COOCH₃.

The monomers of the third group may be used singly or in combinations oftwo or more.

The variety of combinations of monomers constituting the polymer B, andtheir ratio, degree of polymerization, and the like are not particularlylimited. The polymer B contained in the layer A may be a singlecomponent or a combination of two or more. The ion exchange capacity ofthe fluorine-containing polymer B having a carboxylic acid group can beadjusted by changing the ratio of the monomers represented by the abovegeneral formulae (3) and (4).

The thickness of the layer B is preferably 5 μm or more and 50 μm orless, and more preferably 5 μm or more and 20 μm or less. When thethickness of the layer B is within this range, the electrolyticperformance of the ion exchange membrane tend to be more improved, and,as a result, there is a tendency that a higher current efficiency and alower voltage can be achieved. When the membrane thickness of the layerB is within the above range, clusters in the layer B are likely toshrink during electrolysis, and the value of [(the ion cluster diameterof the layer B after electrolysis)/(the ion cluster diameter of thelayer B before electrolysis)] is likely to be small.

In the ion exchange membrane of the present embodiment, from theviewpoint of further improving electrolytic performance and strength, itis preferable that the layer A contains a polymer of a compoundrepresented by the above formula (2), and the layer B contains a polymerof a compound represented by the above formula (3).

In the ion exchange membrane of the present embodiment, the sum of thethickness of the layer A and the thickness of the layer B beforeelectrolysis is preferably 55 μm or more, more preferably 55 μm or moreand 210 μm or less, and even more preferably 85 μm or more and 190 μm orless. When the total thickness of the layer A and the layer B is withinthe above range, the strength of the membrane body tends to be moreimproved. From the same viewpoint, it is preferable that the thicknessof the layer A before electrolysis is 50 to 180 μm, and the thickness ofthe layer B before electrolysis is 5 to 30 μm. Here, the thicknesses ofthe layer A and the layer B mean the thicknesses of the layer A and thelayer B constituting the ion exchange membrane after a hydrolysis step,which will be described below, and before the above-describedelectrolysis, and can be measured by the method described in Examples.The thicknesses can be controlled by, for example, adjusting theextruder capacity and the rate of film take-up in a film forming step,which will be described below.

[Ratio of Ion Cluster Diameters Before and after Electrolysis]

Ion clusters are present in the ion exchange membrane of the presentembodiment in a hydrated state. The ion cluster refers to a space whereions travel and is formed by association of ion exchange groups. The ioncluster diameter varies according to the degree of association of ionexchange groups and the water content of the membrane body, and can becontrolled by the ion exchange capacities of fluorine-containingpolymers and hydrolysis conditions, and, moreover, supplying electricpower may cause the cluster diameter to vary. Due to the ratio of ioncluster diameters before and after electrolysis being within apredetermined range, the ion exchange membrane of the present embodimenthas both excellent membrane strength and excellent electrolyticperformance.

As for the ion exchange membrane of the present embodiment, whenelectrolysis is performed under the electrolytic conditions (1), [(theion cluster diameter of the layer B after electrolysis)/(the ion clusterdiameter of the layer B before electrolysis)] is 0.83 to 0.95,preferably 0.83 to 0.92, and more preferably 0.83 to 0.90. Here, theelectrolysis conditions (1) means that the ion exchange membrane isdisposed between an anode chamber and a cathode chamber, a 3.5 N aqueoussodium chloride solution is supplied to the anode chamber, a 10.8 Naqueous sodium hydroxide solution is supplied to the cathode chamber,and electrolysis is performed for 7 days under conditions having anelectrolysis temperature of 85° C. and a current density of 6 kA/m².Here, “the ion cluster diameter of the layer B before electrolysis”refers to the ion cluster diameter of the layer B in the ion exchangemembrane after the hydrolysis step in the production of the ion exchangemembrane, which will be described below, and before being used inelectrolysis. “The ion cluster diameter of the layer B afterelectrolysis” refers to the ion cluster diameter of the layer B in theion exchange membrane after electrolysis is performed under theelectrolytic condition (1). In the present specification, [(the ioncluster diameter of the layer B after electrolysis)/(the ion clusterdiameter of the layer B before electrolysis)] may be simply referred toas “the ratio of the ion cluster diameters of the layer B before andafter electrolysis”.

When the ratio of the ion cluster diameters of the layer B before andafter electrolysis is 0.83 or more, an increase of voltage duringelectrolysis is suppressed, and deterioration of electrolyticperformance can be suppressed. The reason therefor is considered to be,although there is no intention to limit it to, that the ion clusterdiameter of the layer B before electrolysis is not excessive, and asubstantial increase of thickness due to an increased water content ofthe ion exchange membrane can be suppressed. When the ratio of the ioncluster diameters of the layer B before and after electrolysis is 0.95or less, the ion selectivity during electrolysis is good. The reasontherefor is considered to be, although there is no intention to limit itto, that the ion cluster diameter of the layer B during electrolysisshrinks to an optimum size. From these viewpoints, the ratio of the ioncluster diameters of the layer B before and after electrolysis is 0.83to 0.95. The ratio of the ion cluster diameters of the layer B beforeand after electrolysis can be controlled to the above range in such amanner that, for example, the ratio is lowered by increasing the ioncluster diameter of the layer B before supplying electric power andreducing the ion cluster diameter of the layer B after supplyingelectric power. Specifically, for example, by increasing the treatmenttemperature of a salt exchange treatment, which will be described below,or extending the treatment time, the ion cluster diameter of the layer Bbefore supplying electric power tends to be increased.

[Ion Cluster Diameter]

Before electrolysis, the ion cluster diameter of the layer A in the ionexchange membrane of the present embodiment is preferably 3.0 to 4.5 nm,more preferably 3.2 to 4.0 nm, and even more preferably 3.4 to 3.8 nm.In the ion exchange membrane before electrolysis, the ion clusterdiameter of the layer B is preferably 2.5 to 4.0 nm, more preferably 3.0to 3.8 nm, and even more preferably 3.2 to 3.6 nm. When the layer A iscomposed of two or more layers having different compositions, the ioncluster diameter is defined as the average of their ion clusterdiameters. For example, when the layer A consists of two layers, i.e.,layer A-1 and layer A-2, the average value of the ion cluster diametersof the layer A-1 and the layer A-2 is preferably 3.0 to 4.5 nm. When theion cluster diameters of the layer A and the layer B in the ion exchangemembrane before electrolysis are within the above ranges, there is atendency that the electrolytic performance and strength of the ionexchange membrane are more improved. That is to say, when the clusterdiameters are greater than the lower limits of the above ranges, thereis a tendency that strength is more improved, and when the clusterdiameters are smaller than the upper limits of the above ranges, thereis a tendency that an increase of voltage can be more suppressed. Theion cluster diameters are measured by small angle X-ray scattering(SAXS) after peeling the layer A and the layer B into single-layermembranes consisting solely of the respective layers and impregnatingthe resulting films of the layer A and the layer B with water at 25° C.When the ion exchange membrane has coating layers, SAXS measurement canbe performed in the same manner as above except that the coating layersare removed with a brush, and then the ion exchange membrane isseparated into single-layer membranes consisting solely of therespective layers. Details will be described in Examples below.

As for the ion exchange membrane of the present embodiment, the ioncluster diameter of the layer B after electrolysis under the aboveelectrolysis conditions (1) is preferably 2.0 to 3.3 nm, and morepreferably 2.5 to 3.2 nm. Moreover, as for the ion exchange membrane ofthe present embodiment, from the viewpoint of further improvingelectrolytic performance and strength, it is particularly preferablethat the ion cluster diameter of the layer B before electrolysis is 2.5to 4.0 nm, and the ion cluster diameter of the layer B afterelectrolysis is 2.0 to 3.3 nm.

[Ion Exchange Capacity]

In the ion exchange membrane of the present embodiment, the ion exchangecapacities of the fluorine-containing polymers constituting the layer Aand the layer B are one of the factors that control the ion clusterdiameters. The ion exchange capacity of a fluorine-containing polymerrefers to the equivalent of an exchange group per gram of dried resinand can be measured by neutralization titration. The ion exchangecapacity of the fluorine-containing polymer A constituting the layer Ais preferably 0.8 to 1.2 mEq/g and more preferably 0.9 to 1.1 mEq/g. Theion exchange capacity of the fluorine-containing polymer B constitutingthe layer B is preferably 0.75 mEq/g or more, and more preferably 0.81to 0.98 mEq/g. When the ion exchange capacities of thefluorine-containing polymers are within the above ranges, there is atendency that a decrease of the electrolytic performance and strength ofthe ion exchange membrane is more effectively suppressed. Due to the ionexchange capacity of the fluorine-containing polymer B constituting thelayer B being 0.81 or more, the water content in the ion exchangemembrane is high, and thus clusters are likely to shrink uponelectrolysis. There is a tendency that the larger the ion exchangecapacity of each layer is, the larger the ion cluster diameter of thelayer is, and the smaller the ion exchange capacity is, the smaller theion cluster diameter is. The ion exchange capacity of each layer can becontrolled by, for example, selection of a monomer that constitutes thefluorine-containing polymer contained in the layer and the content ofthe monomer. Specifically, for example, it can be controlled by theratios of monomers of the above general formulae (1) to (3) introduced,and, more specifically, there is a tendency that the larger the contentsof monomers containing ion exchange groups, which are represented bygeneral formulae (2) and (3), are, the larger the ion exchangecapacities are.

[Reinforcement Core Material]

The ion exchange membrane of the present embodiment preferably containsthe reinforcement core materials 3 within the membrane. It is preferablethat the reinforcement core material is capable of reinforcing thestrength and dimensional stability of the ion exchange membrane and ispresent inside the membrane body. The reinforcement core material ispreferably a woven fabric or the like obtained by weaving areinforcement yarn. Since long-term heat resistance and chemicalresistance are necessary, the component of the reinforcement corematerial is preferably a fiber consisting of a fluorine polymer. Thecomponent of the reinforcement core material is not particularlylimited, and examples include polytetrafluoroethylene (PTFE), atetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), atetrafluoroethylene-ethylene copolymer (ETFE), atetrafluoroethylene-hexafluoropropylene copolymer, atrifluorochlorethylene-ethylene copolymer, and a vinylidene fluoridepolymer (PVDF). In particular, a fiber consisting ofpolytetrafluoroethylene is preferably used.

The yarn diameter of the reinforcement core material is preferably 20 to300 deniers and more preferably 50 to 250 deniers, and the weavingdensity (the fabric count per unit length) is preferably 5 to 50counts/inch. The form of the reinforcement core material is wovenfabric, non-woven fabric, a knitted fabric, or the like, and the wovenfabric form is preferable. The thickness of the woven fabric ispreferably 30 to 250 μm, and more preferably 30 to 150 μm.

For the woven fabric or the knitted fabric, examples of the materialinclude, but not limited to, a monofilament, a multifilament, or a yarnor slit yarn thereof, and as for the weaving method, various weavingmethods such as plain weave, leno weave, knitted weave, cord weave, andseersucker are used.

The aperture ratio of the reinforcement core material is notparticularly limited, and is preferably 30% or more, and more preferably50% or more and 90% or less. The aperture ratio is preferably 30% ormore from the viewpoint of the electrochemical properties of the ionexchange membrane, and 90% or less from the viewpoint of the mechanicalstrength of the membrane. The aperture ratio is the ratio of the totalarea (B) where a substance such as an ion can pass in the ion exchangemembrane to the total surface area (A) of the ion exchange membrane, andis expressed as (B)/(A). (B) is the total area of regions in the ionexchange membrane where ions, an electrolytic solution, and the like arenot blocked by the reinforcement core material, the reinforcement yarn,and the like contained in the ion exchange membrane. The method formeasuring the aperture ratio is as follows. A surface image of the ionexchange membrane (a cation exchange membrane before a coating and thelike are applied) is captured, and (B) is determined from the area ofparts where the reinforcement core material is not present. Then, (A) isdetermined from the area of the surface image of the ion exchangemembrane, and the aperture ratio is determined by dividing (B) by (A).

Among these various reinforcement core materials, a plain weaveconfiguration with a weaving density of 10 to 50 counts/inch of, forexample, a tape yarn obtained by slitting a high-strength porous PTFEsheet into a tape form or a highly oriented PTFE monofilament having adenier of 50 to 300, is particularly preferred, and such configurationhaving a thickness in the range of 50 to 100 μm, and an aperture ratioof 60% or more is further preferred.

Furthermore, in the membrane production step, an auxiliary fiber, whichis normally called a sacrifice core material, may be contained in thewoven fabric to prevent yarn slippage of the reinforcement corematerial. Due to the sacrifice core material being contained, thecontinuous holes 2 a, 2 b can be formed in the ion exchange membrane.

The sacrifice core material dissolves in the membrane production step orthe electrolysis environment and is not particularly limited, and rayon,polyethylene terephthalate (PET), cellulose, polyamide, and the like areused. The amount of the sacrifice core material contained in this caseis preferably 10 to 80% by mass, and more preferably 30 to 70% by mass,of the entire woven fabric or knitted fabric.

[Continuous Holes]

The ion exchange membrane of the present embodiment may have thecontinuous holes 2 a, 2 b within the membrane. In the presentembodiment, the continuous hole refers to a hole that can be a flowchannel for cations produced during electrolysis and for an electrolyticsolution. Due to the continuous holes formed, there is a tendency thatthe mobility of alkali ions produced during electrolysis and anelectrolytic solution is more improved. The shape of the continuousholes is not particularly limited, and, according to the productionmethod described below, it can have the shape of a sacrifice corematerial used in the formation of continuous holes.

In the present embodiment, it is preferable that the continuous holespass through the anode side (the layer A side) and the cathode side (thelayer B side) of the reinforcement core material in an alternatingmanner. Due to such a structure, in a part where a continuous hole isformed on the cathode side of the reinforcement core material, cations(such as sodium ions) transported through the electrolytic solution withwhich the continuous hole is filled can flow into the cathode side ofthe reinforcement core material. As a result, the flow of cations is notblocked, and thus there is a tendency that the electrical resistance ofthe ion exchange membrane can be further reduced.

[Coating]

As necessary, the ion exchange membrane of the present embodiment mayhave the coating layers 6, 7 on the cathode side and the anode side,respectively, for preventing attachment of gas. The materialconstituting the coating layers is not particularly limited, and fromthe viewpoint of preventing attachment of gas, it is preferable that aninorganic substance is contained. The inorganic substance is notparticularly limited, and examples include zirconium oxide and titaniumoxide. The method for forming the coating layers is not particularlylimited, and a known method can be used. An example is a methodincluding applying, with a spray or the like, a fluid containing fineparticles of an inorganic oxide dispersed in a binder polymer solution.

[Method for Producing Ion Exchange Membrane]

The ion exchange membrane according to the present embodiment isproduced such that the ratio of the ion cluster diameters of the layer Bcontaining a fluorine-containing polymer having a carboxylic acid groupbefore and after electrolysis under the above electrolytic conditions(1) is controlled to the above range, and, accordingly, the ion exchangecapacities of the fluorine-containing polymer A and thefluorine-containing polymer B, the hydrolysis conditions, and the likeare adjusted. Below, the method for producing the ion exchange membraneof the present embodiment will now be described in detail.

The method for producing the ion exchange membrane of the presentembodiment is not particularly limited, and preferable is a productionmethod including:

1) a step of producing fluorine-containing polymers having ion exchangegroups or ion exchange group precursors that can become ion exchangegroups by hydrolysis (a polymer production step);

2) a step of obtaining a reinforcement core material woven with asacrifice yarn (a reinforcement core material production step);

3) a step of forming the fluorine-containing polymers having ionexchange groups or ion exchange group precursors that can become ionexchange groups by hydrolysis, into a film (a film formation step);

4) a step of forming a composite membrane by embedding the reinforcementcore material and the film (an embedding step); and

5) a step of hydrolyzing the composite membrane with an acid or analkali (a hydrolysis step).

Here, the “ion exchange group” refers to a sulfonic acid group or acarboxylic acid group.

As for the ion exchange membrane of the present embodiment, the ratio ofthe ion cluster diameters of the layer B before and after electrolysiscan be adjusted by, for example, controlling the ion exchange capacitiesof the fluorine-containing polymers in the polymer production step 1)and/or controlling the hydrolysis conditions in the hydrolysis step 5)among the above steps. Hereinafter, each step will now be described.

Step 1) (Polymer Production Step)

The fluorine-containing polymer A having a sulfonic acid group, whichconstitutes the layer A, can be produced by, for example, copolymerizinga monomer of the first group and a monomer of the second group orhomopolymerizing a monomer of the second group, as described above. Thefluorine-containing polymer B having a carboxylic acid group, whichconstitutes the layer B, can be produced by, for example, copolymerizinga monomer of the first group and a monomer of the third group orhomopolymerizing a monomer of the third group, as described above. Thepolymerization method is not particularly limited, and, for example, apolymerization method commonly used for polymerizing fluoroethylene, inparticular tetrafluoroethylene, can be used.

The fluorine-containing polymers can be obtained by, for example, anon-aqueous method. In the non-aqueous method, a polymerization reactioncan be performed, for example, using an inert solvent such as aperfluorohydrocarbon or chlorofluorocarbon in the presence of a radicalpolymerization initiator such as a perfluorocarbon peroxide or an azocompound under conditions having a temperature of 0 to 200° C. and apressure of 0.1 to 20 MPa.

In the production of the fluorine-containing polymers, the variety ofthe combination of the above monomers and the proportions thereof arenot particularly limited, and may be determined according to the kind,the amount, and the like of a functional group that is desired to beimparted to the resulting fluorine-containing polymers.

In the present embodiment, in order to control the ion exchangecapacities of the fluorine-containing polymers, the ratio of thestarting-material monomers mixed may be adjusted in the production ofthe fluorine-containing polymers that form the respective layers.

The fluorine-containing polymer A having a sulfonic acid group, whichconstitutes the layer A, is preferably produced by, for example,polymerizing a monomer represented by the above general formula (2) orcopolymerizing a monomer represented by the above general formula (1)and a monomer represented by the above general formula (2) in thefollowing molar ratio.

Monomer represented by the above general formula (1): Monomerrepresented by the above general formula (2)=4:1 to 7:1

The fluorine-containing polymer B having a carboxylic acid group, whichconstitutes the layer B, is preferably produced by, for example,polymerizing a monomer represented by the above general formula (3) orcopolymerizing a monomer represented by the above general formula (1)and a monomer represented by the above general formula (3) in thefollowing molar ratio.

Monomer represented by the above general formula (1): Monomerrepresented by the above general formula (3)=6:1 to 9:1

Step 2) (Reinforcement Core Material Production Step)

From the viewpoint of further improving membrane strength, areinforcement core material is preferably embedded in the ion exchangemembrane of the present embodiment. In the case of an ion exchangemembrane having continuous holes, a sacrifice yarn is also woven intothe reinforcement core material. The amount of the sacrifice yarncontained in this case is preferably 10 to 80% by mass and morepreferably 30 to 70% by mass of the entire reinforcement core material.It is also preferable that the sacrifice yarn is a monofilament or amultifilament having a thickness of 20 to 50 deniers and consisting ofpolyvinyl alcohol or the like.

Step 3) (Film Formation Step)

The method for forming the fluorine-containing polymers obtained instep 1) into films is not particularly limited, and it is preferable touse an extruder. Examples of the film forming method are as follows.

When the layer A and the layer B constitute respective single layers, anexample is a method including separately forming the fluorine-containingpolymer A and the fluorine-containing polymer B into films.

When the layer A has a two-layer structure consisting of layer A-1 andlayer A-2, examples include a method including forming thefluorine-containing polymer A-2 and the fluorine-containing polymer Binto a composite film by coextrusion, and, separately, forming thefluorine-containing polymer A-1 into a film independently; and a methodincluding forming the fluorine-containing polymer A-1 and thefluorine-containing polymer A-2 into a composite film by coextrusion,and, separately, forming the fluorine-containing polymer B into a filmindependently. Among these, coextrusion of the fluorine-containingpolymer A-2 and the fluorine-containing polymer B contributes toincreasing interfacial adhesive strength, and is thus preferable.

Step 4) (Embedding Step)

In the embedding step, it is preferable that the reinforcement corematerial obtained in step 2) and the films obtained in step 3) areembedded on a heated drum. The reinforcement core material and the filmsare integrated into a single body by being embedded on the drum via agas permeable, heat resistant release paper while removing air betweenthe layers by reduced pressure under a temperature at which thefluorine-containing polymers constituting the respective layers melt,and thus a composite film is obtained. The drum is not particularlylimited, and, for example, is a drum that has a heat source and a vacuumsource, and the surface of which has a large number of microporouspores.

As for the order of laminating the reinforcement core material and thefilms, examples include the following methods depending on step 3).

When the layer A and the layer B are respective single layers, anexample is a method including laminating a release paper, the layer Afilm, the reinforcement core material, and the layer B film on the drumin this order.

When the layer A has a two-layer structure consisting of the layer A-1and the layer A-2, one example is a method including laminating arelease paper, the layer A film, the reinforcement core material, and acomposite film of the layer A-2 and the layer B film on the drum in thisorder; and another example is a method including laminating a releasepaper, a composite film of the layer A-1 and the layer A-2, thereinforcement core material, and the layer B on the drum in this order.

In order to provide projections on the membrane surface of the ionexchange membrane of the present embodiment, the use of a release paperthat has been embossed in advance makes it possible to form projectionsconsisting of molten polymers during embedding.

Step 5) (Hydrolysis Step)

The composite membrane obtained in step 4) is hydrolyzed with an acid oran alkali. In this hydrolysis step, the ratio of the ion clusterdiameters of the layer B before and after electrolysis can be controlledby changing hydrolysis conditions such as solution composition,hydrolysis temperature, and time. In the production of the ion exchangemembrane according to the present embodiment, it is preferable toperform hydrolysis, for example, at 40 to 90° C. for 10 minutes to 24hours in an aqueous solution of 2.5 to 4.0 N potassium hydroxide (KOH)and 20 to 40% by mass of dimethyl sulfoxide (DMSO). Thereafter, it ispreferable to perform a salt exchange treatment under 50 to 95° C.conditions using a 0.5 to 0.7 N caustic soda (NaOH) solution. From theviewpoint of more effectively preventing a voltage increase resultingfrom an excessive increase of the layer thickness, the treatment time ispreferably shorter than 2 hours when the treatment temperature in thesalt exchange treatment is 70° C. or higher.

The ion cluster diameter can be controlled by changing the compositionof the fluid employed in the hydrolysis step, the hydrolysistemperature, the hydrolysis time, and the like. For example, a large ioncluster diameter can be achieved by lowering the KOH concentration,increasing the DMSO concentration, increasing the hydrolysistemperature, or extending the hydrolysis time. By controlling the ioncluster diameter of each layer, the ratio of the ion cluster diametersof the layer B before and after electrolysis can also be controlled, andit is also possible to make the cluster diameter of the layer B afterelectrolysis significantly smaller than that before electrolysis.Specifically, for example, controlling the ion cluster diameter of thelayer B before electrolysis to be large results in a small ratio of theion cluster diameters of the layer B before and after electrolysis.Coating layers may be provided on the surface of the hydrolyzedmembrane.

[Electrolytic Cell]

The electrolytic cell of the present embodiment includes the ionexchange membrane of the present embodiment. FIG. 2 shows a schematicview of one example of the electrolytic cell of the present embodiment.The electrolytic cell 13 includes at least an anode 11, a cathode 12,and the ion exchange membrane 1 of the present embodiment disposedbetween the anode and the cathode. While the electrolytic cell is usablein various types of electrolysis, a case where it is used in theelectrolysis of an aqueous alkali chloride solution will now bedescribed below as a representative example.

The electrolytic conditions are not particularly limited, andelectrolysis can be performed under known conditions. For example, a 2.5to 5.5 N aqueous alkali chloride solution is supplied to the anodechamber, water or a diluted aqueous alkali hydroxide solution issupplied to the cathode chamber, and electrolysis can be performed underconditions having an electrolysis temperature of 50 to 120° C. and acurrent density of 0.5 to 10 kA/m².

The configuration of the electrolytic cell of the present embodiment isnot particularly limited, and may be, for example, unipolar or bipolar.Materials constituting the electrolytic cell are not particularlylimited, and, for example, the material of the anode chamber ispreferably titanium or the like that is resistant to alkali chloride andchlorine, and the material of the cathode chamber is preferably nickelor the like that is resistant to alkali hydroxide and hydrogen. As forthe arrangement of electrodes, the ion exchange membrane and the anodemay be disposed with a suitable space provided therebetween, or theanode and the ion exchange membrane may be disposed to be in contact.While the cathode is generally disposed so as to have a suitable spacefrom the ion exchange membrane, a contact-type electrolytic cell thatdoes not have this space (a zero-gap base electrolytic cell) may beadopted.

EXAMPLES

Below, the present embodiment will now be described in detail by way ofExamples. The present embodiment is not limited to the followingExamples.

The measurement methods in Examples and Comparative Examples are asfollows.

[Method for Measuring Ion Cluster Diameter]

The ion cluster diameter was measured by small angle X-ray scattering(SAXS). As for SAXS measurement, when the ion exchange membrane hadcoating layers, the coating layers were removed with a brush, then thelayer A and the layer B were peeled off, and single-layer membranes eachcomposed solely of either layer were impregnated with water and measuredat 25° C. In SAXS measurement, a SAXS apparatus Nano Viewer manufacturedby Rigaku Corporation was used. Measurement was performed using aPILATUS 100K as a detector with a sample-detector distance of 841 mm fora small-angle area, and using an imaging plate as a detector with asample-detector distance of 75 mm for a wide-angle area, and bothprofiles were combined to obtain scattering data at a scattering anglein the range of 0.1°<scattering angle (2θ)<30°. Measurement wasperformed with 7 samples being placed one on top of the other, and theexposure time was 15 minutes for both small-angle area and wide-anglearea measurements. When data was acquired with a two-dimensionaldetector, data was converted to one-dimensional data by a reasonableprocess such as circular averaging. Correction of error derived from thedetector such as dark current and correction of scattering due tosubstances other than the sample (empty cell scattering corrections)were made on the obtained SAXS profile. When the influence of the X-raybeam shape (the influence of smear) on the SAXS profile was large,corrections (desmear) were also made on the X-ray beam shape. As for theone-dimensional SAXS profile obtained in this way, the ion clusterdiameter was determined in accordance with the technique described byYasuhiro Hashimoto, Naoki Sakamoto, Hideki Iijima, Kobunshi Ronbunshu(Japanese Journal of Polymer Science and Technology) vol. 63, No. 3, p.166, 2006. That is to say, assuming that the ion cluster structure wasrepresented by a core-shell type hard sphere having a particle sizedistribution, and using a theoretical scattering formula that is basedon this model, fitting was performed in reference to the SAXS profile ofa scattering angle region where scattering derived from ion clusters isdominant in the actually measured SAXS profile, to thereby determine theaverage cluster diameter (the ion cluster diameter) and the ion clusternumber density. In this model, the core part was regarded ascorresponding to the ion cluster, and the core diameter was regarded ascorresponding to the ion cluster diameter. The shell layer wasimaginary, and the electron density of the shell layer was regarded asbeing the same as that of the matrix part. Also, the shell layerthickness here was regarded as being 0.25 nm. The theoretical scatteringformula of the model used for fitting is presented below as formula (A).Also, the fitting range was 1.4<2θ<6.7°.

$\begin{matrix}{{{{I_{HS}(q)} = {{{{CNS}\left( {q,a_{2},\eta} \right)}{\int_{0}^{\infty}{{{P(a)}\left\lbrack {{V(a)}{\Phi ({qa})}} \right\rbrack}^{2}{da}}}} + {I_{b}(q)}}}\mspace{20mu} {wherein}\mspace{20mu} {q = {4{{\pi sin\theta}/\lambda}}}\mspace{20mu} {{{S\left( {q,a_{2},\eta} \right)} = \frac{1}{1 + {24{\eta \left\lbrack {{G(A)}/A} \right\rbrack}}}}{G(A)} = {{\frac{\alpha}{A^{2}}\left( {{\sin \; A} - {A\; \cos \; A}} \right)} + {\frac{\beta}{A^{3}}\left\lbrack {{2A\; \sin \; A} + {\left( {2 - A^{2}} \right)\cos \; A} - 2} \right\rbrack} + {\frac{\gamma}{A^{5}}\left( {{{- A^{4}}\cos \; A} + {4\left\lbrack {{\left( {{3A^{2}} - 6} \right)\cos \; A} + {\left( {A^{3} - {6A}} \right)\sin \; A} + 6} \right\rbrack}} \right)}}}}\mspace{20mu} {\alpha = {\left( {1 + {2\eta}} \right)^{2}/\left( {1 - \eta} \right)^{4}}}\mspace{20mu} {\beta = {{- 6}{{\eta \left( {1 + {\eta/2}} \right)}^{2}/\left( {1 - \eta} \right)^{4}}}}\mspace{20mu} {\gamma = {{1/2}{{\eta \left( {1 + {2\eta}} \right)}^{2}/\left( {1 - \eta} \right)^{4}}}}\mspace{20mu} {A = {2{qa}_{2}}}\mspace{20mu} {a_{2} = {a_{0} + t}}\mspace{20mu} {{V(a)} = {\frac{4}{3}\pi \; a^{3}}}\mspace{20mu} {{\Phi ({qa})} = {\frac{3}{({qa})^{3}}\left\lbrack {{\sin ({qa})} - {({qa}){\cos ({qa})}}} \right\rbrack}}\mspace{20mu} {{P(a)} = \frac{{p(a)}/{V(a)}}{\int{{{p(a)}/{V(a)}}{da}}}}\mspace{20mu} {{p(a)} = {\frac{M^{M}}{{\Gamma (M)}a_{0}^{M}}a^{M - 1}{\exp \left( {{- \frac{M}{a_{0}}}a} \right)}}}\mspace{20mu} {M = \left( \frac{\sigma}{a_{0}} \right)^{- 2}}} & {{formula}\mspace{14mu} (A)}\end{matrix}$

Above, C represents a constant; N represents a cluster number density; ηrepresents the volume fraction of a hard sphere, assuming that the core,i.e., the ion cluster part, and the surrounding imaginary shellconstitute a hard sphere; θ represents a Bragg angle; λ represents an Xray wavelength used; t represents a shell layer thickness; a₀ representsan average ion cluster radius, Γ(x) represents a gamma function; and σrepresents the standard deviation of the ion cluster radius (the coreradius). P(a) represents the distribution function of core radius a,where the volume distribution of a follows Schultz-Zimm distributionp(a). M is a parameter representing distribution. Ib(q) representsbackground scattering including scattering derived from excessive waterduring measurement and thermal diffuse scattering, and is assumed as aconstant here. Among the parameters above, N, η, a₀, σ, and Ib(q) arevariable parameters in fitting. In this specification, the ion clusterdiameter means the average diameter of ion clusters (2 a ₀).

[Method for Measuring Thickness of Each Layer after Hydrolysis Step]

The ion exchange membrane after the hydrolysis step and beforeelectrolysis was cut in the cross-sectional direction from the layer A-1side or the layer B side to obtain a portion with a width of about 100μm, and the thickness was actually measured in a hydrated state using anoptical microscope, with the cross section facing upward. At this time,the portion that was cut out was an intermediate part (a valley part)between adjacent reinforcement core materials, the portion measured onthe obtained cross-sectional view, in reference to FIG. 1, is anintermediate part between adjacent reinforcement core materials 3, andthe thicknesses of the layer A and the layer B were measured, with thedirection from (α) toward (β) being regarded as the thickness direction.

[Electrolytic Performance Evaluation]

The electrolytic cells used were zero-gap base electrolytic cellsobtained by modifying the configuration of the electrolytic cell 13shown in FIG. 2 as follows. That is to say, provided were electrolyticcells obtained by modifying the positional relationship of the ionexchange membrane 1 with respect to the anode 11 and the cathode 12 inthe electrolytic cell 13 to attain a state where the ion exchangemembrane 1 and the anode 11 were in contact and a state where the ionexchange membrane 1 and the cathode 12 were in contact (i.e., a“zero-gap” state). Using these zero-gap base electrolytic cells,electrolysis was performed under the following conditions to evaluateelectrolytic performance based on the electrolytic voltage, currentefficiency, and amount of sodium chloride in the produced caustic soda.A case where the ion exchange membrane was in contact with the entireelectrode surfaces of the cathode and the anode as well as a case wherethe ion exchange membrane was in contact with certain points of theelectrode surfaces were both regarded as a zero-gap state.

Brine was supplied to the anode side while adjusting the sodium chlorideconcentration to be 3.5 N, and water was supplied while maintaining thecaustic soda concentration on the cathode side at 10.8 N. Thetemperature of brine was set to 85° C., and electrolysis was performedunder conditions where the current density was 6 kA/m², and the fluidpressure on the cathode side of the electrolytic cell was 5.3 kPa higherthan the fluid pressure on the anode side.

The concentration of sodium chloride contained in caustic soda at day 7of electrolysis was measured by the method of JIS K 1200-3-1. Nitricacid was added to electrolytically produced caustic soda forneutralization, and an iron(III) sulfate ammonium solution andmercury(II) thiocyanate were added to cause the solution to developcolor. The solution was absorptiometrically analyzed with a UV meter tomeasure the sodium chloride concentration in caustic soda, and themeasured value at day 7 was determined as the sodium chlorideconcentration in caustic soda. The UV meter used was a V-630spectrophotometer manufactured by JASCO.

The current efficiency was determined by measuring the mass andconcentration of the produced caustic soda and dividing the amount bymole of caustic soda produced in a specific time by the amount by moleof electrons that flowed during that time.

[Strength Test]

As a strength test, tensile strength and tensile elongation weremeasured in accordance with JIS K 6732 using the ion exchange membraneafter hydrolysis (before electrolysis).

Example 1

As a fluorine-containing polymer A-1, a monomer represented by thefollowing general formula (1) (X₁═F, X₂═F) and a monomer represented bythe following general formula (2) (a=1, b=2, Y═CF₃) were copolymerizedin a molar ratio of 5:1 to give a polymer having an ion exchangecapacity of 1.05 mEq/g. The ion exchange capacity was determined byneutralization titration. The ion exchange capacity was determined inthe same manner in the following Examples and Comparative Examples.

CF₂═CX₁X₂  (1)

CF₂═CF—(OCF₂CYF)_(a)—O—(CF₂)_(b)—SO₂F  (2)

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5.7:1 to give a polymer having an ion exchange capacity of 0.99mEq/g.

As a fluorine-containing polymer B for forming the layer B, a monomerrepresented by the above general formula (1) (X₁═F, X₂═F) and a monomerrepresented by the following general formula (3) (c=1, d=2, Y═CF₃,R═CH₃) were copolymerized in a molar ratio of 7.5:1 to give a polymerhaving an ion exchange capacity of 0.89 mEq/g.

CF₂═CF—(OCF₂CYF)_(c)—O—(CF₂)_(d)—COOR  (3)

More specifically, the fluorine-containing polymers A (A-1, A-2) wereprepared by solution polymerization as presented below.

First, CF₂═CFOCF₂CF(CF₃)O(CF₂)₂COOCH₃ and an HFC-43-10mee solution wereintroduced into a 20 L stainless steel autoclave, and the vessel wasfully replaced with nitrogen, then further replaced with CF₂═CF₂ (TFE),heated until the temperature inside the vessel became stable at 35° C.,and pressurized by TFE.

Then, a 5% HFC43-10mee solution of (CF₃CF₂CF₂COO)₂ as a polymerizationinitiator was introduced to initiate the reaction. At this time,methanol was added as a chain transfer agent. TFE was intermittently fedwhile stirring at 35° C., methanol was added to lower the TFE pressureduring the process, and polymerization was terminated when apredetermined amount of TFE was supplied. After unreacted TFE wasdischarged to the outside of the system, methanol was added to theresulting polymerization solution to aggregate and separate thefluorine-containing polymer. Further, after drying, the polymer A wasobtained. The resulting fluorine-containing polymer was pelletized witha biaxial extruder.

The fluorine-containing polymer B was obtained by the same method as thepolymer A except that CF₂═CFOCF₂CF(CF₃)O(CF₂)₂SO₂F was introduced inplace of CF₂═CFOCF₂CF(CF₃)O(CF₂)₂COOCH₃, no chain transfer agent wasused, and a 5% HFC43-10mee solution of (CF₃CF₂CF₂COO)₂ was added inplace of methanol during the process. Pellets of the fluorine-containingpolymers A and B were also obtained in the same manner in the followingExamples and Comparative Examples.

The resulting fluorine polymer A-2 and fluorine polymer B werecoextruded with an apparatus equipped with 2 extruders, a coextrusion Tdie for 2 layers, and a take-up machine, to give a two-layer film (a1)having a thickness of 93 μm. As a result of observing the cross sectionof the film (a1) under an optical microscope, the thickness of the layerA-2 was 80 μm, and the thickness of the layer B was 13 μm. The layer A-2and the layer B were distinguished by applying polarization. Asingle-layer film (b1) having a thickness of 20 μm for the layer A-1 wasobtained with a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b1), areinforcement core material, and the two-layer film (a1) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane.

A 100-denier polytetrafluoroethylene (PTFE) tape yarn twisted 900times/m into a thread form as the reinforcement core material, a30-denier, 6-filament polyethylene terephthalate (PET) twisted 200times/m as a warp yarn of the auxiliary fiber (sacrifice yarn) and a35-denier, 8-filament PET thread twisted 10 times/m as the weft yarnwere provided, and these yarns were plain-woven in an alternatearrangement such that the PTFE yarn was 24 counts/inch and the sacrificeyarn was 4 times PTFE, i.e., 64 counts/inch, to give a woven fabrichaving a thickness of 100 μm. The resulting woven fabric waspressure-bonded with a heated metal roll to regulate the thickness ofthe woven fabric to 70 μm. At this time, the aperture ratio of the PTFEyarn alone was 75%.

This composite membrane was hydrolyzed at a temperature of 80° C. for0.5 hours in an aqueous solution containing 30% by mass of DMSO and 4.0N of KOH and then subjected to salt exchange treatment for 1 hour under50° C. conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having a primary particle size of 1 μmwas added to the solution in an amount of 40% by mass, and uniformlydispersed with a ball mill to give a suspension. This suspension wasapplied to both surfaces of the hydrolyzed, salt-exchanged ion exchangemembrane by a spray method and dried to thereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.91. Measurement results are shown in Table 1.

Example 2

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) and a monomer represented by the above generalformula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molar ratio of5.7:1 to give a polymer having an ion exchange capacity of 0.99 mEq/g.

As a fluorine-containing polymer B for forming the layer B, a monomerrepresented by the above general formula (1) (X₁═F, X₂═F) and a monomerrepresented by the above general formula (3) (c=1, d=2, Y═CF₃, R═CH₃)were copolymerized in a molar ratio of 7.5:1 to give a polymer having anion exchange capacity of 0.89 mEq/g.

The resulting fluorine polymer A-2 and fluorine polymer B werecoextruded with an apparatus equipped with 2 extruders, a coextrusion Tdie for 2 layers, and a take-up machine, to give a two-layer film (a2)having a thickness of 93 μm. As a result of observing the cross sectionof the film under an optical microscope, the thickness of the layer A-2was 80 μm, and the thickness of the layer B was 13 μm. A single-layerfilm (b2) having a thickness of 20 μm for the layer A-1 was obtainedwith a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b2), areinforcement core material, and the two-layer film (a2) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane. The samereinforcement core material as in Example 1 was used.

This composite membrane was hydrolyzed at a temperature of 50° C. for 24hours in an aqueous solution containing 30% by mass of DMSO and 4.0 N ofKOH and then subjected to salt exchange treatment for 0.5 hour under 90°C. conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having a primary particle size of 1 μmwas added to the solution in an amount of 40% by mass, and uniformlydispersed with a ball mill to give a suspension. This suspension wasapplied to both surfaces of the hydrolyzed, salt-exchanged ion exchangemembrane by a spray method and dried to thereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.93. Measurement results are shown in Table 1.

Example 3

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5.7:1 to give a polymer having an ion exchange capacity of 0.99mEq/g.

As a fluorine-containing polymer B for forming the fluorine-containinglayer B, a monomer represented by the above general formula (1) (X₁═F,X₂═F) and a monomer represented by the above general formula (3) (c=1,d=2, Y═CF₃, R═CH₃) were copolymerized in a molar ratio of 7.3:1 to givea polymer having an ion exchange capacity of 0.91 mEq/g.

The fluorine polymer A-2 and the fluorine polymer B were coextruded withan apparatus equipped with 2 extruders, a coextrusion T die for 2layers, and a take-up machine, to give a two-layer film (a3) having athickness of 93 μm. As a result of observing the cross section of thefilm under an optical microscope, the thickness of thefluorine-containing layer A-2 was 80 μm, and the thickness of thefluorine-containing layer B was 13 μm. A single-layer film (b3) having athickness of 20 μm for the fluorine-containing layer A-1 was obtainedwith a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b3), areinforcement core material, and the two-layer film (a3) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane. The samereinforcement core material as in Example 1 was used.

This composite membrane was hydrolyzed at a temperature of 50° C. for 24hours in an aqueous solution containing 30% by mass of DMSO and 4.0 N ofKOH and then subjected to salt exchange treatment for 1 hour under 95°C. conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having a primary particle size of 1 μmwas added to the solution in an amount of 40% by mass, and uniformlydispersed with a ball mill to give a suspension. This suspension wasapplied to both surfaces of the hydrolyzed, salt-exchanged ion exchangemembrane by a spray method and dried to thereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.86. Measurement results are shown in Table 1.

Example 4

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 6:1 to give a polymer having an ion exchange capacity of 0.95mEq/g.

As a fluorine-containing polymer B for forming the fluorine-containinglayer B, a monomer represented by the above general formula (1) (X₁═F,X₂═F) and a monomer represented by the above general formula (3) (c=1,d=2, Y═CF₃, R═CH₃) were copolymerized in a molar ratio of 8:1 to give apolymer having an ion exchange capacity of 0.85 mEq/g.

The fluorine polymer A-2 and the fluorine polymer B were coextruded withan apparatus equipped with 2 extruders, a coextrusion T die for 2layers, and a take-up machine, to give a two-layer film (a4) having athickness of 100 μm. As a result of observing the cross section of thefilm under an optical microscope, the thickness of thefluorine-containing layer A-2 was 85 μm, and the thickness of thefluorine-containing layer B was 15 μm. A single-layer film (b4) having athickness of 25 μm for the fluorine-containing layer A-1 was obtainedwith a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b4), areinforcement core material, and the two-layer film (a4) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane. The samereinforcement core material as in Example 1 was used.

This composite membrane was hydrolyzed at a temperature of 80° C. for0.5 hours in an aqueous solution containing 30% by mass of DMSO and 4.0N of KOH and then subjected to salt exchange treatment for 1 hour under50° C. conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having a primary particle size of 1 μmwas added to the solution in an amount of 40% by mass, and uniformlydispersed with a ball mill to give a suspension. This suspension wasapplied to both surfaces of the hydrolyzed, salt-exchanged ion exchangemembrane by a spray method and dried to thereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.93. Measurement results are shown in Table 1.

Example 5

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5.7:1 to give a polymer having an ion exchange capacity of 0.99mEq/g.

As a fluorine-containing polymer B for forming the layer B, a monomerrepresented by the above general formula (1) (X₁═F, X₂═F) and a monomerrepresented by the above general formula (3) (c=1, d=2, Y═CF₃, R═CH₃)were copolymerized in a molar ratio of 7.5:1 to give a polymer having anion exchange capacity of 0.89 mEq/g.

The resulting fluorine polymer A-2 and fluorine polymer B werecoextruded with an apparatus equipped with 2 extruders, a coextrusion Tdie for 2 layers, and a take-up machine, to give a two-layer film (a1)having a thickness of 93 μm. As a result of observing the cross sectionof the film (a1) under an optical microscope, the thickness of the layerA-2 was 80 μm, and the thickness of the layer B was 13 μm. The layer A-2and the layer B were distinguished by applying polarization. Asingle-layer film (b1) having a thickness of 20 μm for the layer A-1 wasobtained with a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b1), areinforcement core material, and the two-layer film (a1) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane.

As for the reinforcement core material, a 100-denierpolytetrafluoroethylene (PTFE) tape yarn twisted 900 times/m into athread form was plain-woven to have 24 threads/inch to give a wovenfabric having a thickness of 100 μm. The resulting woven fabric waspressure-bonded with a heated metal roll to regulate the thickness ofthe woven fabric to 70 μm. At this time, the aperture ratio of the PTFEyarn alone was 75%.[0118]

This composite membrane was hydrolyzed at a temperature of 80° C. for0.5 hours in an aqueous solution containing 30% by mass of DMSO and 4.0N of KOH and then subjected to salt exchange treatment for 1 hour under50° C. conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having a primary particle size of 1 μmwas added to the solution in an amount of 40% by mass, and uniformlydispersed with a ball mill to give a suspension. This suspension wasapplied to both surfaces of the hydrolyzed, salt-exchanged ion exchangemembrane by a spray method and dried to thereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.95. Measurement results are shown in Table 1.

Comparative Example 1

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5.7:1 to give a polymer having an ion exchange capacity of 0.99mEq/g.

As a fluorine-containing polymer B for forming the fluorine-containinglayer B, a monomer represented by the above general formula (1) (X₁—F,X₂═F) and a monomer represented by the above general formula (3) (c=1,d=2, Y═CF₃, R═CH₃) were copolymerized in a molar ratio of 8.5:1 to givea polymer having an ion exchange capacity of 0.80 mEq/g.

The resulting fluorine polymer A-2 and fluorine polymer B werecoextruded with an apparatus equipped with 2 extruders, a coextrusion Tdie for 2 layers, and a take-up machine, to give a two-layer film (a5)having a thickness of 93 μm. As a result of observing the cross sectionof the film under an optical microscope, the thickness of the layer A-2was 80 μm, and the thickness of the layer B was 13 μm. A single-layerfilm (b5) having a thickness of 20 μm for the layer A-1 was obtainedwith a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b5), areinforcement core material, and the two-layer film (a5) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane. The samereinforcement core material as in Example 1 was used.

This composite membrane was hydrolyzed at a temperature of 80° C. for0.5 hours in an aqueous solution containing 30% by mass of DMSO and 4.0N of KOH and then subjected to salt exchange treatment for 1 hour under50° C. conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having a primary particle size of 1 μmwas added to the solution in an amount of 40% by mass, and uniformlydispersed with a ball mill to give a suspension. This suspension wasapplied to both surfaces of the hydrolyzed, salt-exchanged ion exchangemembrane by a spray method and dried to thereby form coating layers.

Electrolysis was conducted using the ion exchange membrane obtained asabove. The electrolysis was performed for 7 days at a current density of6 kA/m² at a temperature set to 85° C. in the above-describedelectrolytic cell in which the fluorine-containing polymer layer A wasdisposed to face the anode side. Measured items were the electrolyticvoltage, current efficiency, and amount of sodium chloride in theproduced caustic soda, and were all measured 7 days after the beginningof the electrolysis to evaluate electrolytic performance. At this time,the value of (ion cluster diameter of layer B after electrolysis)/(ioncluster diameter of layer B before electrolysis) was 0.98. Currentefficiency was measured by the same method as in Example 1.

Comparative Example 2

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5.7:1 to give a polymer having an ion exchange capacity of 0.99mEq/g.

As fluorine-containing polymer B for forming the fluorine-containinglayer B, a monomer represented by the above general formula (1) (X₁═F,X₂═F) and a monomer represented by the above general formula (3) (c=1,d=2, Y═CF₃, R═CH₃) were copolymerized in a molar ratio of 7.5:1 to givea polymer having an ion exchange capacity of 0.89 mEq/g.

The resulting fluorine polymer A-2 and fluorine polymer B werecoextruded with an apparatus equipped with 2 extruders, a coextrusion Tdie for 2 layers, and a take-up machine, to give a two-layer film (a6)having a thickness of 93 μm. As a result of observing the cross sectionof the film under an optical microscope, the thickness of the layer A-2was 80 μm, and the thickness of the layer B was 13 μm. A single-layerfilm (b6) having a thickness of 20 μm for the layer A-1 was obtainedwith a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b6), areinforcement core material, and the two-layer film (a6) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane. The samereinforcement core material as in Example 1 was used.

This composite membrane was hydrolyzed at a temperature of 50° C. for0.5 hours in an aqueous solution containing 30% by mass of DMSO and 4.0N of KOH and then subjected to salt exchange treatment for 5 hour under95° C. conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having a primary particle size of 1 μmwas added to the solution in an amount of 40% by mass, and uniformlydispersed with a ball mill to give a suspension. This suspension wasapplied to both surfaces of the hydrolyzed, salt-exchanged ion exchangemembrane by a spray method and dried to thereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.77. Measurement results are shown in Table 1.

Comparative Example 3

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5.7:1 to give a polymer having an ion exchange capacity of 0.98mEq/g.

As a fluorine-containing polymer B for forming the layer B, a monomerrepresented by the above general formula (1) (X₁═F, X₂═F) and a monomerrepresented by the above general formula (3) (c=1, d=2, Y═CF₃, R═CH₃)were copolymerized in a molar ratio of 8.5:1 to give a polymer having anion exchange capacity of 0.80 mEq/g.

The fluorine polymer A-2 and the fluorine polymer B were coextruded withan apparatus equipped with 2 extruders, a coextrusion T die for 2layers, and a take-up machine, to give a two-layer film (a5) having athickness of 93 μm. As a result of observing the cross section of thefilm under an optical microscope, the thickness of thefluorine-containing layer A-2 was 75 μm, and the thickness of the layerB was 15 μm. A single-layer film (b5) having a thickness of 20 μm forthe layer A-1 was obtained with a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b5), areinforcement core material, and the two-layer film (a5) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane. The samereinforcement core material as in Example 1 was used.

This composite membrane was hydrolyzed at a temperature of 75° C. for0.75 hours in an aqueous solution containing 30% by mass of DMSO and 4.0N of KOH and then subjected to salt exchange treatment under 85° C.conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having an average primary particle sizeof 1 μm was added to the solution in an amount of 40% by mass, anduniformly dispersed with a ball mill to give a suspension. Thissuspension was applied to both surfaces of the hydrolyzed,salt-exchanged ion exchange membrane by a spray method and dried tothereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.98. Measurement results are shown in Table 1.

Comparative Example 4

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5.7:1 to give a polymer having an ion exchange capacity of 0.98mEq/g.

As a fluorine-containing polymer B for forming the layer B, a monomerrepresented by the above general formula (1) (X₁═F, X₂═F) and a monomerrepresented by the above general formula (3) (c=1, d=2, Y═CF₃, R═CH₃)were copolymerized in a molar ratio of 8.5:1 to give a polymer having anion exchange capacity of 0.80 mEq/g.

The fluorine polymer A-2 and the fluorine polymer B were coextruded withan apparatus equipped with 2 extruders, a coextrusion T die for 2layers, and a take-up machine, to give a two-layer film (a5) having athickness of 93 μm. As a result of observing the cross section of thefilm under an optical microscope, the thickness of thefluorine-containing layer A-2 was 75 μm, and the thickness of the layerB was 15 μm. A single-layer film (b5) having a thickness of 20 μm forthe layer A-1 was obtained with a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b5), areinforcement core material, and the two-layer film (a5) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane. The samereinforcement core material as in Example 1 was used.

This composite membrane was hydrolyzed at a temperature of 90° C. for0.75 hours in an aqueous solution containing 30% by mass of DMSO and 4.0N of KOH and then subjected to salt exchange treatment under 85° C.conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having an average primary particle sizeof 1 μm was added to the solution in an amount of 40% by mass, anduniformly dispersed with a ball mill to give a suspension. Thissuspension was applied to both surfaces of the hydrolyzed,salt-exchanged ion exchange membrane by a spray method and dried tothereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.97. Measurement results are shown in Table 1.

Comparative Example 5

As a fluorine-containing polymer A-1, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5:1 to give a polymer having an ion exchange capacity of 1.05mEq/g.

As a fluorine-containing polymer A-2, a monomer represented by the abovegeneral formula (1) (X₁═F, X₂═F) and a monomer represented by the abovegeneral formula (2) (a=1, b=2, Y═CF₃) were copolymerized in a molarratio of 5.7:1 to give a polymer having an ion exchange capacity of 0.99mEq/g.

As a fluorine-containing polymer B for forming the layer B, a monomerrepresented by the above general formula (1) (X₁═F, X₂═F) and a monomerrepresented by the above general formula (3) (c=1, d=2, Y═CF₃, R═CH₃)were copolymerized in a molar ratio of 7.5:1 to give a polymer having anion exchange capacity of 0.89 mEq/g.

The fluorine polymer A-2 and the fluorine polymer B were coextruded withan apparatus equipped with 2 extruders, a coextrusion T die for 2layers, and a take-up machine, to give a two-layer film (a5) having athickness of 105 μm. As a result of observing the cross section of thefilm under an optical microscope, the thickness of thefluorine-containing layer A-2 was 80 μm, and the thickness of the layerB was 25 μm. A single-layer film (b5) having a thickness of 20 μm forthe layer A-1 was obtained with a single-layer T die.

On a drum having a heat source and a vacuum source inside and having alarge number of microporous pores in the surface, an air-permeable,heat-resistant release paper, the single-layer film (b5), areinforcement core material, and the two-layer film (a5) were laminatedin this order and integrated into a single body while eliminating airbetween the materials at a temperature of 230° C. under a reducedpressure of −650 mmHg to give a composite membrane. The samereinforcement core material as in Example 1 was used.

This composite membrane was hydrolyzed at a temperature of 80° C. for0.5 hours in an aqueous solution containing 30% by mass of DMSO and 4.0N of KOH and then subjected to salt exchange treatment under 50° C.conditions using a 0.6 N NaOH solution.

A fluorine polymer having a sulfonic acid group, which had an ionexchange capacity of 1.0 mEq/g and were obtained by hydrolyzing acopolymer of CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)O(CF₂)₃SO₂F, was dissolved ina 50/50 parts by mass mixed solution of water and ethanol in an amountof 20% by mass. Zirconium oxide having an average primary particle sizeof 1 μm was added to the solution in an amount of 40% by mass, anduniformly dispersed with a ball mill to give a suspension. Thissuspension was applied to both surfaces of the hydrolyzed,salt-exchanged ion exchange membrane by a spray method and dried tothereby form coating layers.

The electrolysis was conducted using the ion exchange membrane obtainedas above. The value of (ion cluster diameter of layer B afterelectrolysis)/(ion cluster diameter of layer B before electrolysis) was0.97. Measurement results are shown in Table 1.

The compositions, properties, and the like of the ion exchange membranesof the above Examples and Comparative Examples are shown in Table 1.

TABLE 1 Com- Com- Com- Com- Com- Exam- Exam- Exam- Exam- Exam- parativeparative parative parative parative Unit ple 1 ple 2 ple 3 ple 4 ple 5Example 1 Example 2 Example 3 Example 4 Example 5 Layer A-1 Ion mEq/g1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 (Single-layer exchangefilm (b)) capacity Thickness μm 20 20 20 25 20 20 20 20 20 20 Layer A-2Ion exchange mEq/g 0.99 0.99 0.99 0.95 0.99 0.99 0.99 0.98 0.98 0.99(Composite capacity film (a)) Thickness μm 80 80 80 85 80 80 80 75 75 80Layer B Ion exchange mEq/g 0.89 0.89 0.91 0.85 0.89 0.80 0.89 0.80 0.800.89 (Composite capacity film (a)) Thickness μm 13 13 13 15 13 13 13 1515 25 Continuous Yes Yes Yes Yes No Yes Yes Yes Yes Yes hole Structuresof A — 1 1 1 1 1 1 1 1 1 1 fluorine- B — 2 2 2 2 2 2 2 2 2 2 containingC — 1 1 1 1 1 1 1 1 1 1 polymers D — 2 2 2 2 2 2 2 2 2 2 represented byX₁ — F F F F F F F F F F [formula (1), X₂ — F F F F F F F F F F formula(2), Y (in — CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ formula (3)]formula (2)) Y (in — CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ CF₃ formula(3)) R — CH₃ CH₃ CH₃ CH₃ CH₃ CH₃ CH₃ CH₃ CH₃ CH₃ Hydrolysis Temperature° C. 80 50 50 80 80 80 50 75 90 80 Time Hour 0.5 24 24 0.5 0.5 0.5 0.50.75 0.75 0.5 Salt exchange Temperature ° C. 50 90 95 50 50 50 95 85 8550 Time Hour 1 0.5 1 1 1 1 5 1 1 1 Membrane Overall μm 130 117 172 131130 117 230 123 129 141 thickness thickness (after (valley hydrolysispart) step) Layer A μm 115 104 152 116 115 104 202 108 114 116 Layer Bμm 15 13 20 15 15 13 28 15 15 25 Ion cluster (Ion cluster — 0.91 0.930.86 0.93 0.95 0.98 0.77 0.98 0.97 0.97 diameter of layer B afterelectrolysis)/ (Ion cluster diameter of layer B before electrolysis) Ioncluster nm 3.66 3.60 3.64 3.54 3.66 3.60 3.80 3.90 4.04 3.66 diameter oflayer A before electrolysis Ion cluster nm 3.20 3.10 3.46 3.22 3.20 3.043.70 2.50 2.55 3.21 diameter of layer B before electrolysis Ion clusternm 2.90 2.88 2.98 2.99 3.04 2.98 2.85 2.45 2.47 3.11 diameter of layer Bafter electrolysis Electrolytic Current % 97.3 97.5 97.4 97.1 97.3 97.197.4 97.8 97.5 97.8 performance efficiency Voltage V 3.04 3.04 3.04 3.043.04 3.08 3.15 3.07 3.1 3.11 Brine ppm 14 12 12 15 14 15 12 10 13 10concentration in caustic soda Strength Tensile kg/cm 1.80 1.82 1.82 1.821.80 1.80 1.75 1.8 1.81 1.81 strength Tensile % 67 66 66 66 67 66 68 6367 67 elongation

The ion exchange membranes of Examples 1 to 4 had good electrolyticperformance, and also the results of strength evaluation for tensilestrength and tensile elongation showed values at which the ion exchangemembranes can sufficiently withstand electrolysis.

On the other hand, although the ion exchange membrane of ComparativeExample 1 had good results of strength evaluation, the electrolysisvoltage was higher than those of Examples 1 to 4.

Although the ion exchange membrane of Comparative Example 2 had goodresults of strength evaluation, the electrolysis voltage was largelyincreased.

Although the ion exchange membranes of Comparative Examples 3 and 4 hadgood results of strength evaluation, the electrolysis voltage was higherthan those of Examples 1 to 4.

Although the ion exchange membrane of Comparative Example 5 had goodresults of strength evaluation, the electrolysis voltage was higher thanthose of Examples 1 to 4.

The present application is based on a Japanese Patent Application(Japanese Patent Application No. 2015-101292) filed on May 18, 2015, andthe content of which is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The ion exchange membrane of the present invention can be suitably usedin the field of alkali chloride electrolysis.

REFERENCE SIGNS LIST

-   -   1 Ion exchange membrane    -   2 a Continuous hole    -   2 b Continuous hole    -   3 Reinforcement core material    -   4 Layer A    -   5 Layer B    -   6 Coating layer    -   7 Coating layer    -   8 Portion appearing on anode-side surface of layer A    -   α Anode side of electrolytic layer    -   β Cathode side of electrolytic layer    -   11 Anode    -   12 Cathode    -   13 Electrolytic Cell

1. An ion exchange membrane comprising: a layer A comprising afluorine-containing polymer having a sulfonic acid group; and a layer Bcomprising a fluorine-containing polymer having a carboxylic acid group,wherein a ratio of an ion cluster diameter of the layer B afterelectrolysis under the following electrolysis conditions to the ioncluster diameter of the layer B before the electrolysis [(the ioncluster diameter of the layer B after the electrolysis)/(the ion clusterdiameter of the layer B before the electrolysis)] is 0.83 to 0.95:(Electrolysis Conditions) in a zero-gap base electrolytic cell where theion exchange membrane is disposed between an anode chamber to which a3.5 N aqueous sodium chloride solution is supplied and a cathode chamberto which a 10.8 N aqueous sodium hydroxide solution is supplied,electrolysis is performed for 7 days under conditions having atemperature of 85° C. and a current density of 6 kA/m².
 2. The ionexchange membrane according to claim 1, wherein the ion cluster diameterof the layer B before the electrolysis is 2.5 to 4.0 nm; and the ioncluster diameter of the layer B after the electrolysis is 2.0 to 3.3 nm.3. The ion exchange membrane according to claim 1, wherein a sum of athickness of the layer A and a thickness of the layer B before theelectrolysis is 55 μm or more.
 4. The ion exchange membrane according toclaim 1, wherein the ion cluster diameter of the layer A before theelectrolysis is 3.0 to 4.5 nm.
 5. The ion exchange membrane according toclaim 1, wherein a thickness of the layer A before the electrolysis is50 to 180 μm; and a thickness of the layer B before the electrolysis is5 to 20 μm.
 6. The ion exchange membrane according to claim 1, whereinthe layer A comprises a polymer of a compound represented by thefollowing formula (2); and the layer B comprises a polymer of a compoundrepresented by the following formula (3):CF₂═CF—(OCF₂CYF)_(a)—O—(CF₂)_(b)—SO₂F  (2) wherein a represents aninteger of 0 to 2, b represents an integer of 1 to 4, and Y represents—F or —CF₃; andCF₂═CF—(OCF₂CYF)_(c)—O—(CF₂)_(d)—COOR  (3) wherein c represents aninteger of 0 to 2, d represents an integer of 1 to 4, Y represents —F or—CF₃, and R represents —CH₃, —C₂H₅, or —C₃H₇.
 7. An electrolytic cellcomprising the ion exchange membrane according to claim 1.